1. Field of the Invention
The present invention relates generally to image processing for electronic imaging sensors, and more particularly, to an apparatus and method for forming reference pixels in an image sensor such that they behave like imaging pixels, but are insensitive to light, while exhibiting similar dark current characteristics.
2. Description of the Related Art
Optical black pixels (“OB pixels”) are the reference pixels within an imaging sensor pixel array that have virtually identical electrical characteristics as a standard active pixel, yet are insensitive to light. OB pixels are used by various image processing circuits in the sensor or camera to maintain a constant black level independent of gain, illumination, changes in sensor operating conditions, etc. An OB pixel's signal is normally processed using the same signal chain and timing as those that are photo-electrically responsive and form the image of interest. The latter pixels, which constitute the majority of pixels in an imaging sensor, are some times referred to as “Clear Pixels.” While the clear pixels form the image, the OB pixels are only indirectly used in the image formation process.
In an imaging System-on-Chip (iSOC) system, OB pixels are typically used as reference pixels for many calibration operations aimed at enhancing image quality. These include black clamp stabilization, fixed pattern noise suppression and line noise correction. In these and other algorithms, OB pixels must have the identical dark signal response as a clear pixel; they must mimic the response that a clear pixel would have if it were imaging a scene characterized by absence of any light so that the sensor's dark response is accurately set.
Different applications of an imaging sensor pixel array may require differing degrees of image quality. For instance, a CMOS sensor studio broadcast camera used to generated content for HDTV broadcasting (high end application) requires a significantly higher image quality than a CMOS sensor used in a camcorder for consumer applications (medium to low end application). This involves not only better clear pixel quality, but also better quality of optical black. Therefore, the most important property of an OB pixel is how closely it truly represents the dark signal response of a clear pixel under any possible lighting condition of the pixel array. For instance, high end applications require an OB pixel to have significantly higher fidelity under extreme conditions, e.g., higher end applications require OB pixels to maintain a higher degree of opacity than for consumer applications without degrading electrical characteristics.
Early methods for capably reading clear and optically black pixels in MOS sensor arrays are taught in U.S. Pat. Nos. 3,603,731 and 3,822,362. The '362 patent specifically uses design symmetry to eliminate common switching noise in all pixels and, hence, generates a reference signal that mimics and effectively improves the clear signal.
U.S. Pat. No. 4,387,402 discloses the addition of an optically opaque row of detectors in a Charge Injection Device to enhance clear signal quality by differencing the signal voltage between the clear and OB pixels using correlated double sampling. In addition, the OB pixels constitute a reference voltage, or black level, for each column. Nevertheless, those skilled in the art appreciate that the OB pixels are not sufficiently opaque; off-axis light rays will penetrate the overlying metal layer and degrade efficacy.
Kosonocky in U.S. Pat. No. 4,412,343 instead estimates the dark current generated during the integration epoch and teaches means for subtracting the estimate from the total signal. However, no means for accurately determining the OB signal is presented. U.S. Pat. No. 4,602,291 (Temes et al.,) and U.S. Pat. No. 4,819,071 (Nakamura et al.,) also provide circuit means for removing the offset pedestal, but no method for optimally forming the OB pixels. Subsequently, in U.S. Pat. No. 4,678,938 Nakamura also teaches the benefit of OB pixels and the necessity for inclusion along the periphery of the imaging sensor, but no means for forming the OB pixels apart from covering them with a “light shield.”
In U.S. Pat. No. 4,839,729, Ando addresses the challenge of providing a high quality dark reference by reading the dark signals out of a row of light-receiving cells and storing them in a dark signal memory. Unfortunately, it is practically impossible to have sufficiently short integration time to preclude some light pollution in the read cells.
U.S. Pat. No. 4,914,519 further ignores the critical need for forming optimal OB pixels by instead supplying a reference signal that cannot be corrupted by light. While the reference can remove some of the column-related fixed pattern noise and a facsimile of the dark current estimate for the integration epoch, the final image quality is again inadequate since only estimates are used rather than accurate calculations and the reference signal does not change appropriately with sensor operating conditions including temperature. Additional U.S. Pat. Nos. 4,984,085, 5,408,314, 5,408,335, 6,750,910 and 7,098,950 provide alternative circuits for performing the reference subtraction, but no specific solution is disclosed for producing the ideal reference that is best achieved by using properly formed OB pixels.